1. Field of the Invention
The present invention relates to digital-to-analog conversion, and a method of digital-to-analog conversion.
2. Description of the Related Art
Nowadays, thin-film transistor-liquid crystal displays (TFT-LCDs) are the widely used among various flat panel displays.
FIG. 1 is a block diagram illustrating a conventional TFT-LCD panel and peripheral circuits.
An LCD panel 110 may include an upper plate and a lower plate, each including a plurality of electrodes for forming electric fields, a liquid crystal layer between the upper and lower plates, and polarization plates, which are respectively attached to the upper and lower plates, for polarizing light. The brightness of light that is transmitted through a TFT-LCD 100 is controlled by applying corresponding voltages (gray voltages) to pixel electrodes to re-arrange liquid crystal molecules in the liquid crystal layer and provide various gray levels. For applying the gray voltages to the pixel electrodes, a plurality of switching devices, such as TFTs, connected to the pixel electrodes are located on the lower plate of the TFT-LCD panel 110. The switching devices (e.g., TFTs) control the brightness of light through a pixel area and, for color displays, three colors (e.g., R (Red), G (Green), and B (Blue)) can be formed through a pixel array with a color filter arrangement as shown in FIG. 2.
The TFT-LCD 100 includes gate drivers 120 for driving a plurality of gate lines arranged horizontally and source drivers 130 for driving a plurality of source lines arranged vertically. The source and gate lines are arranged on the LCD panel 110 in a matrix configuration. The gate and source drivers 120 and 130 are controlled by a controller (not shown). Generally, the controller is disposed outside the LCD panel 110. The gate and source drivers 120 and 130 are generally disposed outside the LCD panel 110; however, the gate and source drivers 120 and 130 may be disposed on the LCD panel 110 in a chip-on-glass (COG) display.
FIG. 3 is a block diagram illustrating a conventional source driver.
Referring to FIG. 3, the conventional source driver 130 includes a plurality of gamma decoders 131 and buffers 132. Each gamma decoder 131 receives n-bit image data (n=6, 8, 10, . . . ), and selectively outputs an analog voltage corresponding to a digital value of the image data from among 2n analog gray voltages. A graphics card, for example, in the controller processes a three color signal (e.g., red-green-blue (RGB) digital data) to obtain the image data according to the resolution of the LCD panel 110. Analog image signals output from the gamma decoder 131 are buffered by the corresponding buffers 132 and respectively output to source lines S1, S2, S3, etc. The analog image signals output from the buffers 132 quickly charge the source lines S1, S2, S3, etc. and corresponding pixels on the LCD panel 110. Liquid crystal molecules of the pixels receiving the image signals are re-arranged in proportion to applied gray voltages, and thereby control the brightness of light transmitted therethrough.
For enhancing color reproducibility by increasing the number of bits of R, G, and B image data, the area of a gamma decoder circuit used to decode the bits may increase in proportion to the increased number of bits. To avoid such an increase in circuit complexity, an amplifier interpolation scheme has been developed. According to one such amplifier interpolation scheme, representative gray voltages are selected based on upper bits of digital image data and intermediate values are created from the selected representative gray voltages based on the remaining lower bits. The amplifier interpolation scheme may use a half method capable of reducing the gamma decoder circuit area by ½, or a quarter method capable of reducing the area by ¼. In the half method, intermediate interpolated voltages are created from representative gray voltages selected based on the upper bits of input image data. In the quarter method, interpolated voltages with ¼ the interval of representative gray voltages selected based on the upper bits of input image data are created.
This conventional amplifier interpolation scheme depends on input voltages of an amplifier used for interpolation. Interpolation of the voltages may become skewed when differences between input voltages of the amplifier are not small or when the differences are not equal for all gray levels. Accordingly, a source driver that uses the conventional interpolation scheme may not create interpolated voltages that enable generation of stable and uniformly distributed gray level differences. In addition, the source driver requires an offset control device for controlling output differences owing to the self-offset of the amplifier.